SHORT COMMUNICATION NON-CATCH CONTRACTION IN PARAMYOSIN-CONTAINING MUSCLE IN AN ECfflNOTHURHD SEA URCHIN

It has long been known that certain molluscan smooth muscles become stretch resistant and can hold the shell closed for extended periods without fatigue and at low rates of oxygen consumption. This prolonged state of contraction is called catch, and the maintenance of catch force is at least 10 times more economical than the contraction that precedes catch. This mechanism has been extensively studied in the anterior byssus retractor muscle (ABRM) of Mytilus edulis (Riiegg, 1971, 1986). Molluscan catch muscle is structurally characterized by the presence of very long filaments (over 30 fm\), which are much thicker (over 40 nm) than those of skeletal muscle (15 nm). The extra thickness of catch muscle filaments is caused by the presence of paramyosin, which constitutes the core of the thick filaments (Hanson et al. 1957). Based on morphological evidence, it has been suggested that paramyosin is responsible for catch. However, Lowy et al. (1964) proposed a hypothesis in which contractile linkages form between thick and thin filament